1. Field of the Invention
This invention relates to solder reflow techniques in manufacturing processes of printed circuit boards and particularly to a technique of maintaining alignment of parts to be soldered to the solder pads of a printed circuit board by the reflow process.
2. Background and Information Disclosure
Surface mount manufacturing processes of printed circuit boards have evolved to a very complicated technology involving a number of disciplines including mechanics, plastics chemistry, metallurgy, electrochemistry and electronics. The constant drive toward reduced cost has intensified a requirement for more dependable and precise procedures.
The major requirement in fabricating printed circuit parts is to form connections between leads from the various components and terminations on a substrate. This requirement has led to numerous disclosures that depend on the physical characteristics of the parts.
The forerunner of techniques in this field is the well known solder joint. Solder technology has incorporated chemical techniques for precleaning surfaces, metallurgical considerations that have led to optimum solder alloy compositions, heating devices including soldering irons and/or ovens providing accurate control of temperature, etc.
As the size of the components and leads has decreased and the number of leads to be terminated has increased, the approaches to meeting the requirements have become more complex.
Recognition that joining two abutting metal surfaces involves raising the surface energy of the interface of the two surfaces to a point where adhesion of the two metals occurs has led to consideration of the methods in which raising the interfacial surface energy may be achieved.
One method involves putting the two surfaces in contact and raising the temperature of the interface to the melting temperature of one of the metals by simple thermal conduction such as in an oven or with a soldering iron.
Another method of raising the energy of the interface is by sending an electrical current through the interface which is the familiar welding technique.
A less familiar technique is by the use of mechanical vibration known as ultrasonic welding. In the practice of ultrasonic welding, frictional energy is generated by virtue of the two surfaces rubbing one another and the generation of the energy leads to the welding of the two surfaces together. The frequencies used in order to generate sufficiently high interfacial energy is of the order of many kilocycles per second.
For example, U.S. Pat. No. 4,730,764 to Hawkins et al discloses an apparatus for ultrasonically welding a wire to a wire terminal involving an anvil which holds the wire against the terminal while injecting ultrasonic energy into the interface.
U.S. Pat. No. 4,529,115 to Renshaw et al is for a thermally assisted ultrasonic welding apparatus and process in which the interface between parts is preheated then subjected to ultrasonic welding (in the absence of further applied heat). The parts are first preheated to a temperature of 150.degree. F. and then ultrasonically welded. Ultrasonic welding normally generates additional heat so variations in the size of the welds occur as a function of the thermal history of the welding anvil. Therefore, the patent teaches the introduction of a preheating step together with variability of the welding energy in order to obtain a temperature of the surface being welded that is constant from one weld to the next thereby providing uniform welds from one weld to the next.
U.S. Pat. No. 3,791,028 to Missel is for a bonding technique in which a soft metal, preferably gold or lead, is plated onto the surfaces of parts to be bonded. The parts are then clamped together and ultrasonic energy is injected with an anvil. Bonding takes place strictly by virtue of the ultrasonic frictional energy developed. No movement of the parts occurs during the bonding step.
Another important process in surface mount technology is reflow soldering, illustrated in FIG. 1. According to this technique, the exposed copper printed circuits on the surface of a printed circuit board are clad with solder such as by electroplating the tin lead alloy (solder) onto one or both surfaces or by soldering. Another method of applying the solder to the board prior to reflow is to apply the solder as a paste, i.e., a mixture of solder particles in a resinous viscous medium. Then components (integrated circuits, transistors, etc.) are positioned on the board with component leads in contact with the solder pads. Next, the board with components in situ are placed on a conveyor belt and passed through an oven of the type shown in FIG. 1. The board with solder is heated to above the melting temperature of the solder then cooled so that the leads are soldered to the respective solder pads.
A number of precautions must be exercised in implementation of this technique. One problem is oxidation of the surface of the solder and the surface of the part that diminishes wettability of the surface by the solder. Therefore, the practice has been followed to perform the reflow step in a reducing atmosphere. U.S. Pat. No. 5,028,399 to Suppelsa et al discloses use of a catalyst (Ni, Ag, Cu plug) in a manifold delivering reducing gas (formic acid) to effectuate a reduction of the dissociation temperature of the gas so that the gas will be more active as a reducing agent at the reflow temperature.
Another limitation of surface mounting of components involving the reflow step is the accuracy of automatically placing the parts on the board and having them stay in place while the board is transferred to the reflow oven and passed through the reflow zone. Displacement of leads of the components from their respective solder pads is a major limitation of the process, not only in terms of yield, but also in terms of the size of the pad which is feasible to incorporate in design of the board. Production costs are also increased by the necessity of having an operator visually inspect parts as they emerge from the line and then undertake the cost of rework. For optimum reliance of a terminal soldered to a solder pad, the terminal should be located with its center aligned with the center of the pad. In the context of this specification, the terms "aligned" and "centered" are to be understood as meaning that the termination (lead) makes contact with the solder pad in the center of the pad as distal as possible from the edges of the pad.
It will also be understood that the term "solder" applies to alloys used to join metal surfaces by wetting the surfaces with molten solder then allowing the solder to solidify. Solder alloys include tin, lead, silver, gold, indium and other metals known in the art.